A commonly available design for a refrigeration appliance, particularly one for consumer use, includes a cabinet that contains a freezer compartment and a fresh food compartment. These compartments may be arranged e.g., side by side or may include one positioned over the other. In one example of a conventional design, the evaporator portion of a refrigeration loop is positioned in the freezer compartment where a fan moves air in the freezer compartment across the evaporator to freeze the contents of the freezer compartment. A damper positioned between the freezer compartment and the fresh food compartment is used to feed a portion of the air over to the fresh food compartment for cooling its contents.
To control the refrigeration loop that provides cooling for the refrigerator, one previous approach has been the use of a single thermostat such as e.g., a bimetallic thermostat. The compressor operates at a single speed and is cycled on or off based on the set point temperature of the thermostat and the air temperature in the refrigerator compartments.
While control based on the difference between the measured temperature and a temperature set point can be used to effectively control temperature, such methodology provides no direct way to minimize energy consumption of the appliance. For example, in a system based on thermostatic control of a compressor that can only operate at a single speed, cycling the compressor between on and off states may provide more cooling than is needed to reach the desired temperature set point. An overshoot can occur where the single speed compressor removes more heat than is necessary to reach the temperature set point, which results in inefficient energy usage.
A more modern approach to control includes the use of refrigerators having one or more controllers that execute a wide variety of algorithms for temperature control of the appliance. However, many of these controllers are based on complex, non-linear models of the heat transfer behavior of the appliance. Such non-linear controllers typically require a significant amount of tuning based on experimentation that must be performed for each different configuration or model of refrigeration appliance provided.
Additional complexity is encountered in controlling a refrigeration system that utilizes dual evaporators. For example, certain refrigerator appliances may use one evaporator in a fresh food storage compartment and another evaporator in a frozen food storage compartment. Typically, each such compartment has a different set-point temperature and thus each must be separately measured and maintained at the set-point temperature. Because the evaporators are frequently not operated simultaneously, temperature and humidity control in each compartment can be difficult to achieve and gradients may occur.
Therefore, a control system for a refrigerator appliance would be useful. More particularly, a control system for a refrigerator appliance that can determine the speed at which to operate the compressor so as to cool one or more components (including compartments and/or other features) to one or more desired temperature settings while also minimizing the amount of energy usage by the compressor would be useful. Such a control system that is based on a linear model that can be tuned analytically so as to reduce design time would also be useful. A control system that can be used to control a dual evaporator refrigeration system would also be useful.